Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium have flow characteristics that can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electro-magnetomechanical interactive properties for advantageous use in a variety of magnetorheological (MR) devices, such as brakes, clutches, mounts and dampers.
In particular, linear acting MR dampers are commonly used in suspension systems, such as a vehicle suspension system and vehicle engine mounts. PCT patent application 10840, published Jan. 8, 1998 (the '840 application), discloses a proposed linear acting controllable vibration damper apparatus which includes a piston positioned in a magnetorheological fluid-filled chamber to form upper and lower chambers. The piston includes a coil assembly, a core, i.e. pole pieces, and an annular ring element positioned around the pole pieces to form an annular flow passage for permitting flow of the magnetorheological fluid between the chambers. A gas cup or diaphragm is positioned at one end of the cylinder to form a pressurized accumulator to accommodate fluid displaced by the piston rod as well as to allow for thermal expansion of the fluid. When the piston is displaced, magnetorheological fluid is forced through the annular flow passage. When the coil is energized, a magnetic field permeates the annular flow passage and excites a transformation of the magnetorheological fluid to a state that exhibits damping forces.
During assembly of a MR damper, magnetorheological fluid is typically injected into a charging assembly and loaded, along with the piston assembly, into the cylinder forming the damper chambers. If included, accumulator gas and a gas cup or diaphragm must also be injected and loaded into the cylinder. A conventional charging assembly includes a charging tube, a set of fill holes and valves for controlling MR fluid and gas flow through the holes. It has been found that MR fluid leakage can occur in the gap around the valve and corresponding hole during movement of the components from the charging assembly into the damper cylinder. This undesirable leakage can accumulate to significant amounts disadvantageously resulting in unacceptable, expensive MR fluid usage and increased clean-up costs.
Therefore, there is a need for a simple, effective and low cost charging system for charging a MR damper with MR fluid without undesirable leakage.